Embodiments relate to an enhanced method of scheduling Physical Resource Blocks (PRBs) for serving one of a plurality of User Equipments (UEs), a serving cell and/or a wireless cellular network.
Interference
One of the factors limiting the Radio Frequency (RF) coverage of a shared spectrum is co-channel interference. Co-channel interference is a common phenomenon in cellular networks. In Code Division Multiple Access (CDMA) cellular networks like 3GPP2/CDMA2000 and 3GPP/UMTS, the co-channel interference has two components, intra-cell interference and inter-cell interference. In contrast, in Frequency Division Multiple Access (FDMA) based cellular networks like Long Term Evolution (LTE) networks, the uplink (UL) transmission is designed to be orthogonal in the frequency domain between different User Equipments (UEs), thus virtually eliminating the intra-cell interference associated with CDMA. However, LTE UL transmissions are still susceptible to inter-cell interference especially since LTE systems are designed to operate with a frequency reuse factor of one where neighboring cells use the same frequencies. Therefore, in LTE the main constraints on transmission power at different time intervals in a frame (referred to as Physical Resource Blocks (PRBs)) arises from inter-cell interference, making careful management of inter-cell interference particularly important in systems such as LTE.
The UL interference perceived by a cell s is based on interference from the active users u in the neighboring cells s′. Assume that a set of users u using a PRB m in a Transmission Time Interval (TTI) t that contribute to UL interference at a cell s is defined as φm,t,s. The transmit power of an interfering uεφm,t,s for the PRB m in TTI t is Ptm,t,u. The received power of the interfering user u at the serving cell s is Prm,t,u,s=Ptm,t,u·ρm,t,u,s, where ρm,t,u,s is the link gain on the PRB m in TTI t from the interfering user u to the serving cell s. By summing the received power from all interfering users u from the set φm,t,s, the total interfering power on the PRB m in TTI t, as perceived by the serving cell s is given by:
                              I                      m            ,            t            ,            s                          =                              ∑                          u              ∈                              ϕ                                  m                  ,                  t                  ,                  s                                                              ⁢                      Pr                          m              ,              t              ,              u              ,              s                                                          (        1        )            
In LTE, the interference is measured by the performance metric called interference over thermal noise (IoT). Thus, the IoT on the PRB m in TTI t at the serving cell s is:
                              IoT                      m            ,            t            ,            s                          =                                            I                              m                ,                t                ,                s                                      +                                          η                _                            s                                                          η              _                        s                                              (        2        )            where ηs is an averaged long-term noise floor on serving cell s over a 180 KHz wide PRB. The log domain number is expected to be around −118 dBm, assuming a −174 dBm/Hz thermal noise density, minus a 4 dB noise figure of the receiver.
If another user u having a different serving cell s is assigned the same PRB m in TTI t by its own serving cell s, the achieved UL signal to interference plus noise ratio (SINR) of the active user u on the PRB m in TTI t at the cell s is:
                              γ                      m            ,            t            ,            u            ,            s                          =                              Pr                          m              ,              t              ,              u              ,              s                                                          I                              m                ,                t                ,                s                                      +                                          η                _                            s                                                          (        3        )            In equation 3, Im,t(s) represents the interference power on the PRB m in TTI t at the cell s, contributed by all interfering users from the set φm,t,s. Plugging equation (2) into equation (3), results in:
                              γ                      m            ,            t            ,            u            ,            s                          =                              Pr                          t              ,              s              ,              u              ,              s                                                          IoT                              m                ,                t                ,                s                                      ·                                          η                _                            s                                                          (        4        )            
Note that, for simplicity, it is assumed that the serving cell s perceives that all UEs, including interfering UEs and UEs within the serving cell, are in time alignment.
A time averaged per-PRB IoT can be calculated using a single pole IIR filter as: IoTm,t,s=(1−β)· IoTm,t-1,s+β·IoTm,t(s),  (5)where β is a filter coefficient.Scheduling
An enhanced nodeB (eNodeB) in an LTE system is responsible for managing resource scheduling for both uplink and downlink channels. The goal of a resource scheduling algorithm is to optimize allocation of PRBs. UEs are assigned to the PRB based on which UE has the highest priority ratio rm,t,u,s/({tilde over (R)}t,u,s)α, where {tilde over (R)}t,u,v is long-term averaged throughput, rm,t,u,s is instantaneous spectral efficiency and a is a variable used to tune the “fairness” of the scheduler. The average throughput {tilde over (R)}t,u,v on TTI t is computed using the following single-pole IIR filter:{tilde over (R)}t,u,s=(1−β)·{tilde over (R)}t-1,n,s+β·Rt-1,u,s  (6)A scheduler at the serving cell s is configured to assign the PRB m at TTI t to a UE u with the highest determined priority ratio.
In general, there are two types of scheduling algorithms used to determine the priority ratio, opportunistic scheduling and fair scheduling. Fair scheduling focuses on achieving at least a minimum data rate for each UE by setting α=1 for all UEs, while opportunistic scheduling focuses on achieving at least a maximum total data rate among all the UEs serviced by setting α<1 for UEs that have a relatively low throughput.
Limiting Interference
Conventionally, to reduce and maintain the IoT level, several techniques are used including well-known static frequency reuse, limiting maximum transmit power and dynamic techniques that are specific to a LTE/OFDMA system, such as soft fractional frequency reuse (SFFR) and inter-cell interference coordination (ICIC). Inter-cell Interference Control (ICIC) is used to manage IoT in LTE systems for UL transmissions. ICIC scheduling algorithms make use of measurement information to make an informative scheduling decision to limit inter-cell interference. In order to coordinate scheduling in different cells, communication between neighboring cells is required.
Conventional ICIC methods utilize two messages exchanged between eNodeBs over an X2 interface to facilitate coordination of their transmit powers and scheduling of UEs, namely an exchange of an Overload Indicator (OI) and a High Interference Indicator (HII). The OI may be sent from neighboring cells on the X2 interface, as an indication of the average uplink IoT for each PRB at the respective neighboring cell. The OI can take three values, expressing low, medium, and high levels of IoT. The HIT may be sent from the serving cell to the neighbor cell on the X2 interface to indicate that the serving cell will be scheduling uplink transmissions for a cell-edge UE in certain PRBs, and therefore that interference may be high in those frequency regions. The HII is an active message that does not depend on a measurement. Serving cells that receive the OI and HII indicators may then take this information into consideration in scheduling their own UEs to limit the interference impact.
In one or more example embodiments, instead of exchanging merely OI and HII indicators to make informative scheduling decisions, a serving cell may attempt to minimize a cost associated with assigning the PRB to one of the plurality of UEs by determining an enhanced priority ratio. The enhanced priority ratio is determined by exchanging IoT measurements with eNBs in neighboring cells and utilizing these exchanged measurements along with other information to make per-PRB scheduling decisions to control or limit the interference.
The periodicity of the exchanges between eNBs is a configurable parameter and can range from a millisecond timescale (msec) to timescales on the order of minutes (min). Each serving cell s can identify its respective neighboring cells s′ using a 3GPP proposed Self-Optimizing Network (SON) algorithm, such as an Automatic Neighbor Relation (ANR) algorithm. Alternatively, the serving cell s can identify its respective neighbor cells s′ manually based on field or empirical measurements.